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Pioneer CT-F950 Recording Problem

1mike161

1mike161

New Member
My Pioneer CT-F950 has developed a problem when recording onto chrome (CrO2) tapes and with the tape selection switch set to chrome. The recorded level on the left channel is significantly less (about half) than the recorded level on the right channel. The problem does not occur with metal tapes and the tape selection switch set to metal. I have confirmed all this with test tones.



Playing laboratory test tapes shows there is no problem with playback. Output level on both channels is the same. However, when I record test tones onto chrome tapes, the playback level on the left channel is incorrect.



Has anyone encountered this before?
 
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How about on normal bias tapes ? At 333hz test tone are the channels balanced? Then try at 1khz, then 10khz, if the level are decreasing at the higher you go, then look for bias problem. Possibly bias selector switch. If you have a scope you could look for the levels before the bias oscillator signal is injected, to compare the left vs right audio to rule out the signal amps.
 
AudioWizard, I don't think I have any normal bias (standard) tapes, but I will test that type if I can find one. I will perform the test with different frequencies and see what happens. I do have a scope so I can check the level before and after the bias injection. Great tip! Thanks!
 
Test results from measuring recorded output:

Standard Tape & Bias:
333 Hz L = R
1K Hz L = R
10K Hz L = R

Metal Tape & Bias:
333 Hz L = R
1K Hz L = R
10K Hz L > R

Chrome Tape & Bias:
333 Hz L > R
1K Hz L > R
10K Hz L > R



The Following is incorrect. See second post:

Test results from measuring signal prior to bias injection :

333 Hz L = R
1K Hz L = R
10K Hz L > R (0.888mV Left > 0.415mV Right)

I’m not exactly sure how to interpret these test results, but my guess is that there is a problem prior to bias injection. There may be a problem with the bias circuit as well.
 
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I was wrong on the signal measurements prior to bias injection. All frequency tests show the left and right channels are perfectly balanced. I think one of my digital multimeters gave me a bad reading. When I swap meters, the .888mV and .415mV readings also swap. I retested everything with a scope, and confirmed that both channels are balanced prior to the bias circuit.
 
Ok, so from the last reading the problem must be in the bias levels. I would do a few more things first, clean/deoxit the bias select switches, damag the head. Then use the scope and check/compare the bias levels "L" vs "R" between the different tape settings.The bias level will be the highest for metal, next highest Chrome, then normal. If they appear to be equal then swap the wiring on the head right for left, left for right and see if they swap levels.
 
I did not clean/deoxit the bias select switch. I couldn't find easy access to it and I didn't have time to do a lot of dismantling. I did demag the head. After checking the bias output with my scope, I measured an imbalance between the "L" and "R" channels. The "L" channel is half the magnitude of the "R" channel. This measurement was consistent for all bias selections (std, metal, chrome, & FeCr). I assume this consistency rules out a dirty selector switch. So, the problem must be in the bias amp. Right?
 
Sounds like it. Check the bias level before the adjustments to see if there are the same. I don't have a schematic on this model so can't get to the actual component #'s.
 
The bias signal imbalance occurs after the "matching transformers". I'm not sure what these components actually do, but it doesn't look like they are adjustable. The signal difference is half -- one bias channel is half the other in terms of wave magnitude. Any advise on how to proceed? I can't believe the coil/transformer could be bad -- there would probably be no signal at all if it was. There is a 330 pF capacitor right after the coil/transformer. Is the capacitor the more likely culprit?
 
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One thing you might try is to swap the wires to the head, right for left, left for right (channels) to see if the low bias swaps sides. If it doesn't change then at least you know the problem is before this point. When I taught audio repair, I called this method " divide and conquer". Do you have the schematic?, if so can you scan the circuit area in question and send it to me? For me at this point its hard to help you on the details without a schematic diagram, I hope you understand.

Monte
 
I sent you the schematic. Let me know if AK reduces it to the point you can't read it.

By swapping the head channels I would be ruling out (or confirming) the possibility of a bad record head, right? I'm probably in trouble if it's the record head -- I don't think replacements exist.
 
Ok, got the schematic of the bias area, but not the service manual. Yes about swapping the head wires. Just to recap,Using a signal generator as input, are the audio signals the same at IC 1001 pin 9 on both channels( don't ask me why there are IC 1001 pin 9 for both channels, misprint?)? and at the TP's on L203, L204? Bias signal the same at the emitters of Q306, Q307?
 
First, let me thank you for helping me!

I think I figured out how to get to the head wires, so I will try to swap the them this weekend.

I applied a signal generator to the input terminals on the deck. I observed a balanced signal with a dual trace scope at IC1001 pin 9 Left channel and IC1001 pin 9 Right channel. There are two IC1001 chips, each own separate PCB assemblies -- one for each channel, so I don't think there's a misprint here. I measured perfect L/R balance with 333Hz, 1kHz, and 10kHz for all tape type selections (Metal, Standard, Chrome, and FeriChrome). I get the same balanced measurements at the L203 and L204 TP's.

The Bias signal is balanced at the emitters of the Q306 & Q307.

Here is the unbalanced observation:

When I measure the signal at the heads, I get a weird and unbalanced "composite" signal. I assume this composite signal is the result of the Bias signal combining with the audio signal. Anyway, the signal is unbalanced at the heads -- one channel is twice the magnitude of the other. However, there is a service manual procedure to turn off the Bias signal by shorting Q303 to ground. When I do this, the audio signal at the heads is once again balanced.

When I measure the L & R channels of the Bias circuit, everything is good up to the matching transformers L301 & L302. The signal is balanced at the emitters of the Q306 & Q307. The Bias signal becomes unbalanced after L301 & L302. Again, one channel is twice the magnitude of the other.
 
Ok, so if i am understanding correctly, at the junction where C325 and L301 join, there is a difference compared to the same point on the other channel (L302,C326)? Since it appears the audio signal is ok on both channels discontinue the use on audio input at this point so you are looking at only the bias signal. If the above is true there is not a lot of components between this point and the head. Before swapping the head wires, which could be a pain, I would ohm VR304, VR305 and compare the values, if ok then swap the C325,C326,if no change then swap the head wires. L301. L302 could be a problem but I doubt it, but I never say never.
 
Yes, the unbalanced signal occurs at the junction of C325 & L301 compared to the junction of C326 & L302. And, yes, I did not use an input signal when checking the bias circuit. I will try the "divide and conquer" method on the suspect components, saving the head for last.

Do I need to remove VR304 & VR305 from the circuit to check their values? Also, I think C235 & C236 (330 pF caps) might be Polystyrene Film type caps. Do I need to be concerned with heat here?

Thanks so much!
 
What I would do is pull up one lead of the C325, C326, measure the VR's ( they are set up as rheostats, wiper and one leg tied together) If ok, then I would then swap the caps, one leg already pulled. Depending on your skill level, I wouldn't be concerned with heat if done quickly, but if you want to be safe, heat sink the leads with like a hemostat. The electron gods are with you!
 
Last night, I measure the VR's without taking anything out of the circuit. They are supposed to be 33k ohm, and they both measured between 1 ohm and 30k ohms. So, I think they're OK unless you think the caps must be lifted from the circuit to get an accurate measurement. I am comfortable swapping the caps -- I think I can move quickly enough. I will swap the caps this weekend.
 
I decided to swap the head channels, left for right, since I figured out how to get to those wires. And...the problem reversed. Now the right channel has the problem. So, I have a bad record head.:( I guess I'm dead in the water now -- I don't think it will be easy to find a replacement head. Anyway, thanks again for your help!

Mike
 
Man thats too bad. You might look for other models 900,1250, etc. The head mounting is not special, fairly standard compared to say a Nak. Also might look at other makes that are 3 head decks. Glad I could help at least finding the problem.
 
The 950 still works great as a player and I don't record that much anyway. I have a CT-F750 I can record with if I need to. I was really just trying to see if I could fix it. I have had pretty good luck repairing amplifiers over the years and thought I could take on a tape deck. And, technically, I did fine the problem -- I just have to find the replacement part now. :)
 
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